DIY American cheese from Colby/Cheddar?

So I've been reading SE for a while, always salivating at Kenji's work, but I live in Australia, where it's a bit harder to find American cheese. I have easy access to Colby and Cheddar though, which according to Wikipedia, was what American cheese used to be made out of. So I was wondering, does anyone know what a good blend percentage would be, if I were to embark on blending my own American cheese?